1. Field of the Invention
The present invention relates generally to the geothermal field and more particularly to methods for conducting flows of hot, geothermal brine, water or steam especially such hot geothermal fluids which are highly corrosive in composition.
2. Discussion of the Prior Art
Large, subterranean reservoirs of naturally occuring geothermal steam or water, including brine, exist in many regions of the world. Such geothermal reservoirs are most commonly found in regions of high volcanic, fumarole or geyser activity, as exists along the Pacific Ocean rim, of which California forms a portion.
Because of the typically high temperature, large geothermal steam or water reservoirs typically contain vast amounts of thermal energy. To reduce use of, and dependence on, fossil fuels which can be expected to be eventually depleted, interest exists in the use of geothermal energy for the production of electric power. And, in fact, geothermal steam at The Geysers region of Northern California has, for several years, been used to generate, at competitive rates, at least about two percent of California's electric power requirements. Accordingly, geothermal steam energy at the Geysers displaces daily in excess of about 33,000 barrels of oil, which would be required to generate an equivalent amount of power.
Recently the greatly increased cost of petroleum and the actual or potential disruptions of petroleum supplies from, for example, the Middle East, has caused increased interest in the utilization of geothermal energy to produce power. Although geothermal steam can readily be used to generate electric power, commercially practical sources of geothermal steam are relatively uncommon and most large, accessible sources have already been or are being developed. As a result, substantial effort has been and is being directed to development of economical processes and apparatus for the use of the much more commonly found geothermal water or brine to generate electricity.
General processes whereby geothermal water or brine can be used to generate electric power are, of course, well known. It is known, for example, that geothermal water or brine having a temperature above about 400.degree. F. and a natural pressure of several hundred psig can be flashed to a reduced pressure whereby some of the water or brine is converted into steam. In turn, the steam so produced is used to drive steam turbine-generators. Lower temperature geothermal water or brine can, in contrast, be used in a binary fluid system to vaporize a low boiling point liquid, the gas formed being used to drive gas turbine generators. In some places, geothermal water or brine is, in fact, being used in such manners to generate electric power.
Very often, however, electricity generated by geothermal water or brine, and in particular by geothermal brine, is not cost competitive with electric power generated, for example, by use of fossil fuels. In general, the problems causing the relatively high power generating costs relate to the very large flow rates of geothermal brine required to produce reasonable amounts of power and the chemically complex composition and characteristics of the brine. For example, since a geothermal brine flow rate in excess of one million pounds per hour may be required to produce sufficient steam for generating about 10 megawatts of power, large and hence, expensive, brine handling equipment, including piping, fittings, valves and flashing vessels, is required.
One problem associated with the complex composition of many geothermal brines is the scaling of brine handling equipment by various impurities, such as silica, dissolved into the brine from the underground formations from which the brine is extracted and which precipitate from the brine as brine equilibrium conditions are changed. Considerable costs are, therefore, frequently associated with equipment descaling and/or controlled removal from the brine of scale forming materials so as to reduce equipment scaling.
Another brine composition problem associated with geothermal brine power production (and other brine uses) relates directly to the highly corrosive nature of many geothermal brines, such problems being compounded by the large brine flow rates usually required to produce electric power in commercially useful amounts. The corrosive nature of many geothermal brines (as well as of some geothermal water and steam) is at least partially due to dissolved chlorides and such gases as hydrogen sulfide and carbon dioxide, but is generally rendered much more chemically complex by the brine typically containing many other materials, some of which typically act as buffering agents. These impurities cause many brines to be highly acidic brine pH's of about 4.5 to about 5.5 being common.
Corrosion of down-hole brine handling equipment has been found to be particularly severe and costly in many geothermal brine extraction wells. This is because in such brine production wells, which may typically have depths of 2000 feet to 6000 feet, and especially in lower regions thereof, brine temperatures are higher and pH's are usually lower than in above ground brine handling equipment. An additional problem associated with production well piping is mechanical stresses, at high brine temperatures, principally by the suspended weight of the production pipe. Such stresses can cause mechanical failure of the pipe, especially of pipe that has been weakened by corrosion.
To minimize costs, most geothermal brine production well bores are lined with relatively inexpensive, non-corrosion resistant low-carbon steel pipe. In some regions of the production well bore, there may be several concentric low carbon steel liners, the inner liners extending to progressively greater depths. Thus, in the wellhead region there may be several concentric liners, whereas, at lowermost depths there may be only a single bore liner (or none at all). Ordinarily, the annular regions between these liners is filled with cement (concrete).
Corrosion protection of the well bore liners is typically provided by a corrosion-resistant production pipe which is suspended from the wellhead inside the innermost liner. An inert gas, such as nitrogen, is typically introduced into the annular space between the production pipe and the innermost bore liner under sufficient pressure to maintain the space free of the geothermal brine down to a depth, for example, of about 1500 feet. In such circumstances, the brine is kept from contact with most of the liner pipe by the inert gas "plug".
If, however, the production pipe corrodes through in the region of the inert gas "plug," the gas will leak from the production pipe liner space and geothermal brine, by its natural pressure, will fill the space and will tend to cause rapid corrosion of the liner. Since down-hole brine pressure may be in excess of 1000 psig, when the innermost bore liner corrodes through, the brine may also be forced outwardly through the underlying cement and thus may attack the next-innermost liner. The brine may also be forced upwardly through the cement and so cause wellhead leakage. In cases of severe corrosion, wellhead integrity may be lost and the well-head may be blown off by the brine pressure. Because of various detrimental impurities in many geothermal brines, brine spillage caused by significant wellhead leakage or blown-off wellheads can result in very high clean up costs.
It can also be appreciated that the replacement of corroded production pipe and/or the relining of well bores is very costly, not only due to the cost of replacement pipe but also because of the cost of bringing in a drill rig required for the operation. The cost of replacing the production pipe in a typical brine extraction may, therefore, be as high as $250,000.
Although corrosion problems associated with above ground brine handling equipment downstream of the wellhead are usually less severe than down-hole corrosion problems, such corrosion is still often sufficiently severe to dictate the use of comparatively expensive, corrosion-resistant materials.
In attempts to find suitable high strength, corrosion-resistant materials, many different types of metal alloys, including chrome-moly alloys, nickel alloys, stainless steel alloys, and various titanium alloys have been tested in brine flows and/or have been experimentally used for the construction of brine production pipe or of test lengths thereof. Heretofore it has ordinarily been found that those alloys which provide good corrosion resistance either do not have, or do not retain, sufficient high temperature strength or cannot be economically used to construct pipe and related fittings and equipment in the desired large diameters and sizes enabling economical brine extraction rates.
Various nickel alloys, for example, appear to provide good resistance to brine corrosion but require cold working to achieve high strength. As a result, pipe and fittings larger than about 9-10 inches in diameter are extremely difficult and costly to produce from such alloys. As another example, highly pure (about 99.7 percent) titanium has been found to be resistant to corrosion by geothermal brine but does not provide needed high temperature strength. Chrome-moly alloys (for example, 9 chromium, 1 molybdenum) have, on the other hand, usually been found to provide good high temperature strength, but have also been found to be very poorly resistant to corrosion by typical geothermal brines.
More satisfactory alloys, in terms of corrosion resistance, high temperature strength and relative ease of fabricating large sizes of piping and equipment, are, therefore, needed for many geothermal brine extraction and handling systems in order to reduce system procurement and operating costs to an extent enabling the production of competitively priced electric power.
It is, therefore, an object of the present invention to provide a method for conducting a flow of corrosive geothermal brine (or of other geothermal fluid) which resists corrosion while at the same time providing good high temperature strength and ease in fabricating large size piping and equipment.
Another object of the present invention is to provide a method for conducting a flow of corrosive geothermal fluid in which a beta-alpha titanium-base alloy is used to construct piping and brine handling equipment.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following description.